Closed-loop pre-coding technology has been introduced in the Long Term Evolution (LTE) Release 8 (Rel-8) system to improve the spectrum efficiency. Closed-loop pre-coding firstly requires the same set of pre-coding matrices, referred to as a codebook, to be stored at both a base station and a user equipment. The user equipment estimates channel information from a common pilot signal of a cell and then selects a pre-coding matrix from the codebook according to some criterion which can be maximizing mutual information, maximizing output signal to interference and noise ratio, etc. The user equipment feeds back an index of the selected pre-coding matrix in the codebook to the base station over an uplink channel, where the index is referred to as a Pre-coding Matrix Indicator (PMI). The base station can determine the pre-coding matrix to be used for the user equipment from the value of the received index. The pre-coding matrix reported by the user equipment can be considered as a quantified channel state information.
In an existing cellular system, an array of antennas of a base station is typically arranged horizontally as illustrated in FIG. 1 and FIG. 2. A beam at a transmitter of the base station can be adjusted only horizontally but with a common vertical down tilt angle for every user equipment, so various beam-forming/pre-coding technologies are generally applied based upon horizontal channel information. In fact, a radio signal propagates in three dimensions in space, and the performance of the system may not be optimized with this common vertical down tilt angle. Adjusting of the beam in vertical domain may be of great significance to the improvement of the performance of the system. Along with the development of antenna technologies, an array of active antennas with each array element being separately controllable has emerged in the industry as illustrated in FIG. 3A and FIG. 3B. Dynamic adjusting of a beam in vertical domain becomes possible with this array of antennas. 3D beam-forming/pre-coding may be performed in a Frequency Division Duplex (FDD) system based upon channel state information reported by a user equipment, possibly using a codebook as conventionally used in the LTE Rel-8 system. However the existing codebook is designed for horizontal beam-forming/pre-coding, so a direct application thereof to 3D beam-forming/pre-coding may degrade the performance.
In summary, the existing codebook is designed for horizontal beam-forming/pre-coding, so a direct application thereof to 3D beam-forming/pre-coding may degrade the performance.